speedometer help/info

[Roger Coen]

My 67 has 185 Michelin radials because that's the only red-stripe ones they make. It had them when I bought the car 3 years ago. Question is; my speedos slow, what's the problem? Car has 3:55's and I've installed and reinstalled the correct color gear just to make sure. Is it the smaller tires, speedo bad, different gears than the case says it is or what? Any ideas?

THX, Roger

[Jack H.]

Re: speedometer help/info

Probably a combination of things....

First, all tires with the same generic profile rating (say, 7.75x15, what your car originally shipped with) are NOT identical in geometry! There was a nice Restorer article on rubber a few years back that documented minor height/width differences between supposedly identical tires. So, there's one source of discrepancy.

Next, you've changed from the factory original tire (hooray! the bias ply tires are TERRIBLE to drive on and only good, in my opinion, for concours showing) and come into the world of radials. The Restorer article talked to this point showing you have to go mfgr/model at a time and compare the exact geometry of the target tire to that of the original tire and documented SIGNIFICANT deltas here....

Last, you didn't say whether you had automatic or manual tranny, but I'll assume you've got a 4-speed as this was the most popular configuration. You'll find you have to match the drive gear (inside the Muncie tail housing) to the driven gear (pop-in piece you're describing) to get proper calibration. You can only go a modest way in calibrating the speedo before you're forced to drop the tranny, open the tail housing and find out what the heck's inside from the owner(s) before you....

Of course, if you don't want to go to this expense/effort to solve the mystery (you don't say how far off cal you are....), you can call around to your local speedo shops and find a pro who will build you an out-board calibration box (figure $80-150) to put you dead nuts on (FOR THOSE WHEELS, TIRES and THAT REAR END).

[grr]

Re: speedometer help/info

Roger, you tried the gear and no change but have had same probem since your purchase assume. Need more info for me and members. 1- Have you pulled/inspected the cable assy.If in doubt change it out. 2- How many miles do you have that are original on your car. 3- Is that original cable. 4- Does your odometer work. 5- 4 speed or auto. 6- Car ever been restored.(usually rebuild or renew as necessary speedo)and how long ago. 7- Do you drive the car a lot. Thanks,grr.#33570

[Iron Duke NCRS #22045]

185 what?

185/70-15? What is the model of Michelin tire? What is the tooth count on the currently installed driven gear? A shorter tire will make the speedo read high (relative to actual speed), not low.

Duke

[Wayne M.]

[Roger Coen]

Re: speedometer help/info

1. it reads SLOWER 2. 185x70'S 3. BY 'red' do you mean purple? 4. odo works but slow also 5. what colors are the other gears? 6. 4 speed

did I miss anything?

Thanks for the help guys

[Iron Duke NCRS #22045]

Re: speedometer help/info

7.75-15 = 775 revs/mile: 185/70-15 = 834 revs/mile

nominal speedo error = 834/775 or 7.6 percent: ergo at 60 MPH true, speedo should read 64.56. This assumes all other parts in the system are nominal.

Three possibilities I can think of

Wrong speedo driven gear - parts book says brown, 18T for 3.55 w/7.75-15

Wrong speedo drive gear - parts book says 8T (no color code listed)

Wrong speedo head calibration - GM cars have a spec, X MPH per 1000 cable revs (don't remember the exact speed), but this can be changed by altering magnetic flux on speedo magnet. I have had this done on a GM car. Perhaps this was done to yours. Altering mag flux will alter speedo reading, but not odo, since it is driven directly off the cable.

Assuming everthing was nominal with 185/70 installing the next higher gear tooth count (19T) would reduce high reading to only about 2 percent. Possibly a higher tooth count geat - 20 or 22 teeth is installed. This may be the most likely possibility since both speedo and odo is low. If speedo head has not been altered and trans has an 8T drive gear, then a 19T (natural)driven gear should get you to about two percent error. Any closer will require an adaptor.

Duke

[Iron Duke NCRS #22045]

Correction

I misread the parts book. 67 driven gear with 3.55/7.75x15 should be 23 teeth (black) not 19(my mistake), but Wayne gets 21 (?). 25 tooth (orange) should get speedo/odo within two percent with short tires,

[Wayne M.]

Re: Driven Speedo Gear Correction

Duke --- IMO there's obvious errors, at least in my 1970 parts book (Bubba's brother Billy-Joe responsible). For instance '67s with 7.75 rubber and 3.55 as well as 3.70 both called to use the red 21 tooth sender, which is impossible, for these ratios have a 5% spread. Then, they call for a 23 tooth black gear for 3.73(?) [no tire size indicated], and 25 tooth orange for a 4.10(?) and 7.75's.

Roger is best to count the teeth, for dirty trans lube on a yellow (24 teeth), green (22 teeth) or natural (19 teeth) gear can skew the color impression. That said, there are two "families" of sender gears; the 18 to 21's are of a slightly larger (+0.1") diameter than the 22 to 25 tooth family. To accomodate this, there are two different 8-tooth DRIVE gears for the output shaft. The larger plastic gears will not fit in the adapter of a trans with the drive gear intended for the smaller driven gears (higher numerical rear drive ratios). The opposite (smaller dia driven gears in a trans intended for the larger gears) will work for a while, but eventually the driven gear wears and misses/slips and quits. If the car has a trans from an originally equiped 4.56 rear end, then the DRIVE gear is different again, with only 6 teeth, and this gear requires a 20 or 21 tooth driven gear of a "third family" to get reasonable accurate speedo readings with a 4.56 diff.

In '65/66 w/7.75's, it's generally accepted that the following diffs = driven gears, and I don't see why a 7.75 tire on a '67 should require 1 tooth more, unless there were field complaints of speedos reading lower than true. 3.08 = brown(18); 3.36 = blue(20); 3.55 = red(21); [second family] 3.70 = green(22); 4.11 = yellow(24). The 19 / 23 / 25 tooth driven gears were there to provide fine tuning for tire size differences.

So Roger, count your driven gear teeth, and while you're at it, verify your final diff drive (driveshaft rotations for each tire rotation). Could well end up being a miscalibrated speedo head, but this would probably give a non-constant error.

[Dale Pearman]

Go to a Spedo Shop-Rated G

There's a major cool speedometer shop in Memphis where a converter box is added to your transmission gear output. You drive in and they calibrate things to 1%.I haven't used the service but heard about it from JOE RAY at JOE RAY PARTS CO. He's got a web page at www.joerayparts.com. I would suppose that other shops around the country would also provide this service. Contact Joe for info.

Varooom!

[Wayne M.]

Re: Speedo Gear Correction (Oops)

Gotta cut back on these late night tech posts --- Low numerical final (diff) ratios take the group of SMALL colored plastic gears [~0.78" dia.] (brown, neutral, blue, red), and these are the ones that tend to slip/strip when inserted into the tailshaft adaptor of a trans that has a drive gear for the larger plastic driven gears [0.87" dia.] (green, black, yellow, orange) = high numerical final drive diffs.

[Iron Duke NCRS #22045]

Zapped by the part catalog, again

Excellent summary, Wayne. My 9/77 parts catalog has the same inconsistencies/errors. I couldn't find the red 21T gear because I was looking at the 3-speed section last night, not the 4-speed section. The four-speed section lists '67 with 3.55/7.75 twice showing both a 21T and 22T driven gear. I figure 21T is correct and a 22T would mostly compensate for the shorter tires, but there's something else amiss in this case if the speedo reads low.

Duke

[Roger 30751]

Re: speedometer help/info

Thanks for the help! Think I will count the revs (drvsft vs tire), count the gear teeth and play with different speedo gears first. If that doesn't work I might get different radials because the car is a little low anyway. Any suggestions there? Redlines would be first choice.

[Iron Duke NCRS #22045]

Re: speedometer help/info

How about blackwalls and some real grip. GY Eagle GT4s are about as good you as you can get to fit the production wheels, and the 215/70HR-15 matches the revs/mile figure of the OEM 7.75-15. You can always make a mask and get some red paint.